Technical Field
The present disclosure relates to a method and system for compensating systematic non-linearities in a signal supplied by a capacitive inertial sensor, in particular an inertial micro-electromechanical (MEMS) sensor, such as for example an accelerometer.
Description of the Related Art
Known in the prior art are inertial-measurement units or systems typically comprising an acceleration sensor (accelerometer) having one or more (e.g., three) sensing axes X, Y, Z, designed to measure movements (accelerations) to which the accelerometer is subject during use with respect to the Earth's reference system. Other inertial measurement systems, such as gyroscopes, are available in the prior art.
MEMS technology has favored miniaturization of accelerometers. Schematically and by way of example, an inertial sensor of a known type includes one or more fixed parts (also referred to as fixed masses, or stators) and a mobile mass (rotor). The rotor is capacitively coupled to the stators so that it forms one or more capacitors with each stator. In other words, the stators and the rotor form the respective plates of one or more capacitors. The signal of variation of capacitance of said capacitors indicates a displacement of the rotor with respect to the stator and generates the output signal of the inertial sensor, which indicates the acceleration to which the rotor is subject during use of the accelerometer.
At the end of the manufacturing steps, the inertial sensor is calibrated so for making up, at least in part, for systematic errors generated by the manufacturing process. In particular, a factor that affects the output signal of the inertial sensor is the misalignment of the mobile mass (rotor) with respect to the ideal position that it should occupy with respect to the fixed parts (stators). In particular, according to a known embodiment, stator electrodes (e.g., two stator electrodes) constitute as many plates of respective capacitors, whereas a rotor electrode constitutes a common plate of said capacitors; in this case, the rotor electrode is spatially arranged between the stator electrodes.
In ideal manufacturing conditions, the plate that forms the rotor electrode is spaced at equal distances apart from the plates that form the stator electrodes so that the respective capacitors show, in conditions of rest, a same value of capacitance. However, in real cases, there may exist an undesirable misalignment on account of which said capacitors show, in conditions of rest, a different value of capacitance. When the inertial sensor operates as differential capacitive sensor, the output signal is given by the difference of variation of capacitance of the two capacitors formed by the stator electrodes with the rotor electrode. It is evident that, in the case of the aforementioned manufacturing errors, an undesirable misalignment of said electrodes causes a nonzero output signal also in conditions of rest and further introduces a deterioration of the performance of nonlinearity of the output signal. In particular, in the presence of a marked initial misalignment of the position of the rotor towards the stators, for high values of acceleration there is a markedly nonlinear behavior of the output signal (of a parabolic type). Said behavior is undesirable in the majority of applications in which accelerometers are used.